Let a gesture name the manual section that shows it

The help sheet says which move does a thing, and the manual has a
picture of the thing being done, but nothing joined the two: a user
reading "Pinch it with two fingers" had no way from there to the GIF of
it.

A GESTURE tag takes an optional `manual:` field naming a heading of
docs/manual/README.md by its anchor. The scan checks every one against
the anchors the bundled page is rendered with and fails when the manual
has no such heading, so renaming a section cannot leave the sheet
linking to the top of the page; gestures-check carries the same failure
into CI. The anchor goes into gesture_book.rs as a new field, and into
docs/gestures.md as a "See it" link to manual/README.md#anchor. The help
sheet draws a "See it" button beside the title of each gesture that has
one, which opens the bundled manual at that section.

The field is additive: a tag without it is unchanged, and no gesture
carries one yet.

tools/traceability/src/gestures.rs

@@ -57,6 +57,15 @@
 //! field before it, which is what lets `why` run to a sentence. An unrecognised
 //! key is an error rather than a silently dropped line: `pointr:` should not
 //! quietly cost a gesture its desktop half.
+//!
+//! # `manual:`, the section that shows it
+//!
+//! Optional. It names a heading of `docs/manual/README.md` by its anchor —
+//! `manual: looking-closer`, with or without the `#` — and the help sheet then
+//! offers "See it", opening the bundled manual there, where a picture shows
+//! the move being made. The anchor must exist: [`check_manual`] fails the scan
+//! when the manual has no such heading, so renaming a section cannot leave a
+//! link on the sheet that opens the manual at the top.
 
 use std::collections::BTreeMap;
 
@@ -79,6 +88,8 @@ pub struct Gesture {
     /// a user consulting the help sheet wants the gesture, and a reader of the
     /// document wants the argument.
     pub why: Option<String>,
+    /// The manual section that shows it, as a heading anchor without `#`.
+    pub manual: Option<String>,
     pub file: String,
     pub line: usize,
 }
@@ -105,7 +116,7 @@ impl Gesture {
 }
 
 /// A key that may appear in a gesture block.
-const KEYS: &[&str] = &["where", "touch", "pointer", "keys", "why"];
+const KEYS: &[&str] = &["where", "touch", "pointer", "keys", "why", "manual"];
 
 /// Something wrong with a tag, reported rather than dropped.
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -211,6 +222,7 @@ pub fn extract_from_text(text: &str, path: &str) -> (Vec<Gesture>, Vec<GesturePr
             pointer: take("pointer"),
             keys: take("keys"),
             why: take("why"),
+            manual: take("manual").map(|m| m.trim_start_matches('#').to_string()),
             file: path.to_string(),
             line: start + 1,
         };
@@ -285,6 +297,29 @@ fn split_key(body: &str) -> Option<(String, String)> {
     Some((key.to_ascii_lowercase(), tail.to_string()))
 }
 
+/// Every `manual:` that names a section the manual does not have.
+///
+/// `anchors` is [`crate::manual::anchors`] of the README — the ids the bundled
+/// page gives its headings — so this and the page cannot disagree about what
+/// exists.
+pub fn check_manual(gestures: &[Gesture], anchors: &[String]) -> Vec<GestureProblem> {
+    gestures
+        .iter()
+        .filter_map(|g| {
+            let m = g.manual.as_ref()?;
+            (!anchors.iter().any(|a| a == m)).then(|| GestureProblem {
+                file: g.file.clone(),
+                line: g.line,
+                what: format!(
+                    "gesture `{}` names manual section `#{m}`, which docs/manual/README.md \
+                     has no heading for",
+                    g.title
+                ),
+            })
+        })
+        .collect()
+}
+
 /// Group gestures under their `where:`, keeping the order the sections were
 /// first seen so the document reads in the order a user meets them.
 pub fn by_section(gestures: &[Gesture]) -> Vec<(String, Vec<&Gesture>)> {
@@ -339,6 +374,11 @@ pub fn render_markdown(gestures: &[Gesture]) -> String {
             for (modality, how) in g.routes() {
                 m.push_str(&format!("- **{modality}** — {how}\n"));
             }
+            if let Some(anchor) = &g.manual {
+                m.push_str(&format!(
+                    "- **See it** — [in the manual](manual/README.md#{anchor})\n"
+                ));
+            }
             if let Some(why) = &g.why {
                 m.push_str(&format!("\n{why}\n"));
             }
@@ -381,6 +421,8 @@ pub fn render_rust(gestures: &[Gesture]) -> String {
     r.push_str("    pub touch: &'static str,\n");
     r.push_str("    pub pointer: &'static str,\n");
     r.push_str("    pub keys: &'static str,\n");
+    r.push_str("    /// The manual section that shows it, a heading anchor; empty for none.\n");
+    r.push_str("    pub manual: &'static str,\n");
     r.push_str("}\n\n");
 
     r.push_str("/// Every gesture, in the order the sections were first seen in the source.\n");
@@ -402,6 +444,10 @@ pub fn render_rust(gestures: &[Gesture]) -> String {
                 "        keys: {},\n",
                 quote(g.keys.as_deref().unwrap_or(""))
             ));
+            r.push_str(&format!(
+                "        manual: {},\n",
+                quote(g.manual.as_deref().unwrap_or(""))
+            ));
             r.push_str("    },\n");
         }
     }
@@ -590,6 +636,38 @@ pub fn zoom() {}
         assert_eq!(g[1].title, "Second");
     }
 
+    /// `manual:` is read, with or without its `#`, and a section the manual
+    /// does not have is a problem rather than a link to the top of the page.
+    #[test]
+    fn a_manual_anchor_must_name_a_section_the_manual_has() {
+        let text = "\
+// GESTURE: Zoom
+//   where: Develop
+//   touch: Pinch
+//   manual: #looking-closer
+// GESTURE: Pan
+//   where: Develop
+//   touch: Drag
+//   manual: looking-further
+";
+        let (g, p) = extract_from_text(text, "x.slint");
+        assert!(p.is_empty(), "{p:?}");
+        assert_eq!(g[0].manual.as_deref(), Some("looking-closer"));
+        let anchors = crate::manual::anchors("## Looking closer\n");
+        let problems = check_manual(&g, &anchors);
+        assert_eq!(problems.len(), 1, "{problems:?}");
+        assert!(problems[0].what.contains("looking-further"));
+        assert_eq!(problems[0].line, 5);
+    }
+
+    #[test]
+    fn a_manual_anchor_is_linked_from_the_document_and_carried_in_the_table() {
+        let text = "// GESTURE: Zoom\n//   where: Develop\n//   touch: Pinch\n//   manual: looking-closer\n";
+        let (g, _) = extract_from_text(text, "x.slint");
+        assert!(render_markdown(&g).contains("(manual/README.md#looking-closer)"));
+        assert!(render_rust(&g).contains("manual: \"looking-closer\","));
+    }
+
     #[test]
     fn sections_keep_the_order_they_were_first_seen() {
         let text = "\